fixed no channel icons when using tvheadend (#5996)

Co-authored-by: Cody Robibero <cody@robibe.ro>
Co-authored-by: Maximilian Wehrstedt <wehrstedt.coding@outlook.de>

MediaBrowser.Providers/Manager/ProviderManager.cs

@@ -167,7 +167,7 @@ namespace MediaBrowser.Providers.Manager
                 throw new HttpRequestException("Invalid image received.", null, response.StatusCode);
             }
 
-            var contentType = response.Content.Headers.ContentType.MediaType;
+            var contentType = response.Content.Headers.ContentType?.MediaType;
 
             // Workaround for tvheadend channel icons
             // TODO: Isolate this hack into the tvh plugin